Air-to-air heat pumps and refrigeration systems often must operate under conditions that cause frost to form on the evaporator. To prevent build up of frost, which causes coil blockage and loss of capacity, such systems must have a method of periodically defrosting the coil. Most modern heat pump systems (as well as refrigeration systems) employ a reverse cycle defrosting scheme whereby the refrigeration circuit is reversed to melt frost, snow, and ice from the coil. Various methods of determining when to initiate and when to terminate the defrosting cycle have been employed, from simple time based schemes to complex demand schemes. A true demand method is preferable because it causes the system to defrost only when necessary, thereby improving system efficiency and availability.
One demand scheme centers around measurement and use of the temperature of the outdoor coil liquid line, (for short, liquid line temperature (LLT)). This temperature will drop as frost forms on and begins to block the coil. However, the LLT is sensitive also to a number of other parameters such as outdoor dry bulb temperature (ODT), outdoor wet bulb temperature, outdoor coil airflow, indoor temperature, indoor coil airflow, and system speed. A number of schemes have been employed for adapting the defrosting logic to changing weather conditions for a fixed speed system using the LLT and the rate of change of the LLT, as in U.S. Pat. No. 4,590,771, Jacob E. Shaeffer et al, and U.S. Pat. No. 4,563,877, James R. Harnish.
These schemes appear to be inadequate when applied to a variable speed system because of the effects of many other parameters, especially system speed, on the LLT. FIG. 8 shows how the difference between the LLT and the ODT varies under normal operating conditions at various systems speeds.
Ideally a defrosting system would turn on only when the frost buildup had reduced the system efficiency by a certain percentage, and would remain on only until the frost had been removed. Various control methods and apparatus have been devised for that purpose.
U.S. Pat. No. 4,751,825, issued Jun. 21, 1988, to Roger Voorhis et al. for Defrost Control For Variable Speed Heat Pumps points out that known methods of determining the degree of frost buildup on the coil include using photo-optical techniques, sensing the speed of the fan, and measuring the difference in the refrigerant pressure between the inside and the outside coil. All of them have disadvantages. Another approach, employed in some demand defrost systems, comprises sensing the temperature differences between the coil and the ambient air and, when the difference reaches a predetermined level, initiating the defrost cycle. This requires two sensors, typically thermistors. Those available at reasonable cost have inherent differences such that when a pair are used, it is necessary to conduct a calibration process for each individual system, which can be time consuming and expensive. Some other types of sensors are reasonably accurate without calibration, but are too expensive to use in an adaptive defrost system.
The Voorhis et al. patent discloses an adaptive defrost system for a variable speed heat pump wherein the time between defrosts is continuously updated by multiplying the last time between defrosts by a ratio of the desired and actual differences between the pre-defrost and after-defrost saturated coil temperatures. The same thermistor is used for both pre and after-defrost measurements, so calibration is not required.
The compressor speed is measured at only one point during the defrost cycle, however, and that only for the purpose of storing it in memory to return to the same speed after running the compressor at maximum speed during the defrosting period. The compressor must operate at this specified speed until the system reaches a steady state condition so that the appropriate saturated coil temperature measurement can be made. During this time period, the system is not capable of operating at the speed necessary to meet the desired load commanded by the thermostat.
The present invention is not so limited. It is based on different principles, and provides substantial improvements and advantages over the known prior art.